The present invention relates to a method of analyzing an iron melt for producing compacted graphite iron, comprising the steps of: receiving thermal data from cooling of a casted melt comprising a predetermined amount of carbon, magnesium, balance iron and unavoidable impurities, plotting the temperature of the cast melt against time such that a plotted time-temperature curve is generated, and comparing the generated plotted curve to at least one reference curve, said reference curve representing a corresponding thermal analysis of another melt, the resulting nodularity of which is known, for the purpose of predicting the nodularity of the cast melt on basis of the difference between said curves.
The present invention also relates to a method of producing compacted graphite iron, comprising the steps of: providing a melt comprising a predetermined amount of carbon, magnesium, balance iron and unavoidable impurities, casting at least a part of the melt in a mold, and performing a thermal analysis on the cast melt during cooling thereof in accordance with the analysis method of the invention.
The invention also relates to: a computer program comprising program code means for performing all the steps of the inventive analysis method when said program is run on a computer; a computer program product comprising program code means stored on a computer readable medium for performing all the steps of the analysis method of the invention when said program product is run on a computer, and; a computer system for implementing the analysis method of the invention comprising a processor operable to run a computer program according to the invention.
Cast irons are differentiated by the shape of the graphite particles. Grey cast iron is characterized by randomly oriented graphite flakes, while the graphite in ductile iron exists as individual spheres. The graphite particles in CGI are randomly oriented and elongated as in grey iron, but they are shorter, thicker and have rounded edges.
In comparison to either grey iron or ductile iron, the entangled compacted graphite clusters interlock themselves into the iron matrix to provide a strong adhesion. This graphite shape suppresses crack initiation and propagation and is the source of both the increased mechanical properties relative to grey iron and the improved thermal conductivity relative to ductile iron. These advantageous properties of CGI have made it a suitable material for cylinder blocks of internal combustion engines, in particular diesel engines. High titanium content CGI is also commonly used in exhaust manifold and power steering pump parts thanks to a very good thermal conductivity and high-temperature strength.
The graphite microstructure of Compacted Graphite Irons is expressed in terms of percent nodularity. For simultaneous optimization of mechanical properties, castability, machinability and thermal conductivity, the graphite should be controlled within 0-20% nodularity specification range (more than 80% of the graphite particles must be in the compacted/vermicular form) in all performance-critical sections of the casting. Flake graphite is not permitted. The nodularity percentage and the ferrite/pearlite matrix structure can be evaluated by so called chart comparison technique or by image analysis, both of which are standardized and well known to the person skilled in the art.
It is well-known in the present field of technology that the content of magnesium as an alloying element in the iron melt is of vital importance to the formation of nodules in the cast material and the resulting CGI microstructure. However, not only the Mg but also other alloying elements are involved in the process that leads to the generation of CGI. For example, graphite nodularity after magnesium treatment is affected by low initial sulfur level and this in turn may lead to nodular graphite formation at lower residual magnesium in the treated iron. Accordingly, increasing the sulfur addition, for the same magnesium addition, could be one possible way of promoting the formation of CGI for a given melt. However, it should be born in mind that magnesium is still regarded as the key element in order to obtain CGI, and fine calibration of the Mg content is a very efficient way of affecting the formation of CGI.
When the nodularity is to be predicted and controlled by means of the addition of magnesium, the amount of sulfur and oxygen plays an important role to the amount of magnesium that will be needed. Normally, the sulfur content in the melt is possible to control by a precise addition thereof. The content of oxygen is, on the other hand, not so easy to control or to monitor. Therefore, there will always be a certain degree of uncertainty of how much magnesium that will be required in order to obtain a specific nodularity.
In order to predict the nodularity for a specific melt prior art suggests a plurality of different methods. One such method is disclosed by Y. X. Li, Q. Wang; “Intelligent evaluation of melt iron quality by pattern recognition of thermal analysis cooling curves” J. Mat. Proc. Tech. 161 (2005) pp. 430-434. This method is based on the assumption that two melts with identical cooling curves, i.e. curves in which the temperature of the cast melt is plotted against time, and melt composition will also result in identical microstructures. The method presented by Li and Wang is a calculation of curve similarity in which a comparison of a plotted curve and at least one reference curve comprises measurement of temperature difference for predetermined times, and comparison of curve shape of said curves, and weighing together differences obtained by said comparison in order to present a difference value Ω on which the prediction of said nodularity is based. However, the method does not take into consideration that there may be variations in a) initial (pouring) temperature of the melt, b) the thermal cup filling ratio, and c) the carbon equivalent between the two castings that are compared to each other (the carbon equivalent of the melt being expressed as CE=C+Si/4+P/2 or, as an alternative, CE=C+(Si+P)/3, where C is mass % carbon, Si mass % sulfur and P is mass % phosphor). The present inventors have, however, realised that, due to said variations, also curves that, as analysed with the method of Li and Wang, have a rather different Ω may result in irons of very similar or almost identical nodularity.
It is desirable to present an alternative method of analyzing an iron melt by means of which an improved prediction of the nodularity of the cast melt compared to that of prior art is obtained, as well as an alternative method of producing compacted graphite iron with the aid of said analysis method.
According to a first aspect of the invention, in the initially defined method for analyzing an iron melt, said comparison on which the nodularity is predicted is performed along each of said curves for a time interval t1−t2 corresponding to a temperature interval T1−T2, where T1 is in the range of TEstan−TEmin, where TEstart is the temperature of beginning formation of graphite in the melt and TEmin is a minimum temperature before start of eutectic recalescence in the melt, and T2 is in the range of Tsolidus−(Tsolidus−20° C.), and that other time intervals in said curves are excluded from said comparison. It has been found that it is the given range of t1−t2 that gives a significant contribution to the analysis, and that further regions of the curves outside said range may, because of differences in the carbon equivalent between the analysed melt and the references melt, provide an important contribution to the calculated Ω value that, however, is less important to the nodularity than previously expected. By eliminating said further ranges from the analysis, the effect of different carbon equivalents is thus taken into consideration and differences in a calculated Ω that are due to such differences are thus excluded from the analysis.
According to a second aspect of the invention in the initially defined method for analyzing an iron melt, any of said curves is multiplied with a time factor such that the length of the curves expressed as t2 minus t1 becomes the same. Because of differences in the casting conditions, typically differences in the thermal cup filling ratio, the total time from the start of the casting to the end thereof, when the melt has solidified and reached a certain temperature, may vary between the analysed melt and the reference melt and thereby contribute to an incorrectly high Ω value that is not representative for the factual differences between the two melts. By means of the hereby suggested measure, the contribution to the Ω value caused by different casting conditions is thus eliminated or at least suppressed.
According to a preferred embodiment of said second aspect of the invention, the comparison on which the nodularity is predicted is performed along each of said curves for a time interval t1−t2 corresponding to a temperature interval T1−T2, where T1 is in the range of TEstart−TEmin, where TEstan is the temperature of beginning formation of graphite in the melt and TEmin is a minimum temperature before start of recalescence in the melt, and T2 is in the range of Tsolidus−(Tsolidus−20° C.), and that other time intervals in said curves are excluded from said comparison. Accordingly this is a combination of the first aspect and the second aspect. By the combination thereof, variations in the carbon equivalent as well as in the thermal cup filling ratio are compensated for in the analysis, and detrimental effects on a calculated Ω value caused by such variations are avoided.
According to one embodiment of the invention T2 is in the range of Tsolidus−(Tsolidus−10° C.), and according to a preferred embodiment T2 is Tsolidus. It has been found that the contribution to a (value from differences between different castings for the time range after that Tsolidus has been reached do not give any particular improvement of the analysis result but may rather have a negative impact thereon. Therefore, it is preferred not to base the analysis on parts of the plotted curves that reflect the time after that Tsolidus has been reached.
According to one embodiment, T1 is TEmin—The advantage of choosing this point as the starting point of the comparison interval is that it is reasonably easy to detect in the plotted curve and that, up to that point, possible differences between the plotted curve of the analysed melt and of any reference melt is of less importance to the prediction of the nodularity, as already described above.
According to yet another embodiment, T1 is TEstart—It should be mentioned that TEstart is in reality very close to TEmin as seen along the time axis in the plotted curve. Thus, the technical effect in terms of suppression of the effect of variations in carbon equivalent is not so different from that of having TEmin as a starting point. Likewise to TEmin, also TEstart has the advantage of being relatively easy to detect on a typical thermal analysis curve for a cast iron. TEstart is a defined by a local minimum in the first order derivative of the temperature, and by the second order derivative thereof being zero, i.e. defining a point of inflexion. By means of polynomial-adapted data, this inflexion point is relatively easy to detect.
According to one embodiment of the invention, any of said plotted curve and said reference curve is shifted along its time-axis such that it is equal for the two curves. In other words, shifting of at least one of the curves along the time axis is performed such that tr of the reference curve is located at the same position along the time axis as of the plotted (and analysed) curve. Thereby, regard is taken to the fact that there may be differences in initial (pouring) temperature of the analysed melt and the reference melt, and that the time when TEmin or TEstart occurs for the respective sample might differ and that, initially, t1 of curves are therefore displaced relative to each other along the time axis. Preferably, this step is to be taken after the establishment of the interval t1−t2 by which the curves of the analysed melt and the reference melt are adapted to each other in accordance with the teaching of the invention. Advantageously, this step is followed by the step in which any of said curves is multiplied with a time factor such that the length of the curves expressed as t2 minus t1 becomes the same.
According to one embodiment of the invention, said plotted curve is compared to a plurality of different reference curves for melts of different final nodularity, and the predicted nodularity is chosen to be the known nodularity of the reference curve that is defined as least different from the plotted curve. This technique is advantageous in those cases in which there is a large number of reference curves to compare with, such that there is a rather good chance of finding a curve that is very similar to the one of the analysed melt. The actual determination of the difference between the curves may be any one that is suitable for implementation in a computer program. It may, alternatively, be an ocular determination, preferably made by any experienced operator.
According to one embodiment, the comparison of the plotted curve and said at least one reference curve comprises measurement of temperature difference for predetermined times, and comparison of curve shape of said curves, and weighing together differences obtained by said comparison in order to present a difference value Ω on which the prediction of said nodularity is based.
According to one embodiment said melt consists of, in mass %:
C 3.0-4.0 preferably 3.55-3.80
Si 1.8-4.0 preferably 1.9-2.2
Cu 0-1.0 preferably 0.8-1.0
Mo 0-0.3
Mn 0.3-0.5
P 0-0.03
S 0.006-0.015
Sn 0.04-0.07
Cr 0-0.10
Tl 0-0.015
Mg 0.005-0.020 preferably 0.008-0.015
Ni 0-0.05
balance Fe and unavoidable impurities.
Melts with such a composition are advantageous for the purpose of producing a compacted graphite iron with nodularity and further mechanical properties that will be acceptable and advantageous for many applications.
Preferably, the carbon equivalent of the melt, expressed as CE=C+Si/4+P/2 where C is mass % carbon, Si mass % sulfur and P is mass % phosphor, is in the range of 4.0-4.4%.
Another aspect of the invention is also achieved by means of the initially defined method for producing a compacted graphite iron, which is characterized in that that the content of a nodularity-affecting agent in a remaining part of said melt, that has not been yet cast, is altered as a response to the predicted nodularity being outside a predetermined range, or that the content of a nodularity-affecting agent in a second melt, the characteristics of which corresponds to the characteristics of the cast melt as regards composition, casting temperature and carbon equivalent, is altered as a response to the predicted nodularity being outside a predetermined range. In other words, the analysis according to the present invention is taken advantage of in a production process, wherein the content of a nodularity-affecting agent in a melt is controlled on basis of the nodularity prediction for the melt on which the analysis is performed. The term “nodularity-affecting” could also be referred to as “vermicularity-affecting”.
Preferably, the content of said nodularity-affecting agent in said remaining part of the melt or in said second melt is altered to such a level that a predicted nodularity of the melt, now comprising said altered content of the nodularity-affecting agent, is within said predetermined range. How much of the nodularity-affecting agent that is to be added to the melt may be decided on basis of prior data from further reference melts or by means of any suitable calculation method by means of which the desired amount is predicted. Possibly, or even preferably, a further test casting and thermal analysis in accordance with the teaching of the present invention is performed after addition of said nodularity-affecting agent in order to establish whether the requested nodularity within said predetermined range has been obtained. If needed, further adjustment of said nodularity-affecting agent is made, followed by yet a test casting and analysis, until a predicted nodularity within said range is obtained.
According to one embodiment, said nodularity-affecting agent is magnesium, Mg. Other possible agents the content of which could be altered as a response to the result of the nodularity prediction are cerium, calcium and/or titanium. Normally, the nodularity agent is added in order to increase the nodularity. However, if the nodularity is too high, a nodularity-affecting agent that suppresses the nodule generation or counteracts the nodularity-affecting effect of, for example, Mg could be added.
According to yet another embodiment, the amount of Mg in said melt is increased if the nodularity is below a predetermined threshold value.
The invention also relates to a computer program comprising program code means for performing one or more, preferably all, the steps of the inventive analysis method when said program is run on a computer.
The invention also relates to a computer program product comprising program code means stored on a computer readable medium for performing one or more, preferably all, the steps of the inventive analysis method when said program product is run on a computer.
Furthermore, the invention also relates to a computer system for implementing the analysis method of the invention, comprising a processor operable to run a computer program according to the invention.
Further features and advantages of the present invention will be presented in the following detailed description of an embodiment thereof.